Compounds that inhibit the biosynthesis of L-asparagine and thymidylate are antitumor agents. The mechanisms of action of certain known antitumor drugs which may act as specific inhibitors of L-asparagine synthetase and orotidine 5'-phosphate decarboxylase (the enzyme responsible for catalyzing the last step in the biosynthesis of uridylate, which is needed to produce thymidylate) will be investigated. The mechanisms of these two enzymes will be studied; chemical models will be used to clarify the enzyme chemistry. Based on the determined mechanisms of these enzymes, specific irreversible inactivators -affinity labeling compounds and suicide inactivators - will be designed and synthesized, then tested as antitumor agents. These inactivators also will be used to determine the amino acid sequence and catalytically active residue at the active sites of the enzymes.